The present invention relates to a white, biaxially oriented, flame-retardant polyester film comprising at least one layer which comprises a polyester and a cycloolefin copolymer (COC). The invention further relates to the use of the polyester film, and to a process for its production.
White, biaxially oriented polyester films are known from the prior art. These known prior art films are either easy to produce, have good optical properties or have acceptable processing performance. DE-A 2 353 347 describes a process for producing milky polyester film having one or more layers, which comprises preparing a mixture from particles of a linear polyester with from 3 to 27% by weight of a homopolymer or copolymer of ethylene or propylene, extruding the mixture as a film, quenching the film and biaxially orienting the film via orientation in directions running perpendicular to one another, and heat-setting the film. A disadvantage of this process is that regrind which arises during production of the film (essentially a mixture of polyester and ethylene or propylene copolymer) cannot be reused without yellowing the film. However, this makes the process uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the roughness of the film is much too high, and this gives the film a very matt appearance (very low gloss), undesirable for many applications.
EP-A 0 300 060 describes a single-layer polyester film which comprises, besides polyethylene terephthalate, from 3 to 40% by weight of a crystalline propylene polymer and from 0.001to 3% by weight of a surface-active substance. The effect of the surface-active substance is to increase the number of vacuoles in the film and at the same time to reduce their size to the desired extent. This gives the film greater opacity and lower density. A residual disadvantage of the film is that regrind which arises during production of the film (essentially a mixture of polyester and propylene homopolymer) cannot be reused without yellowing the film. However, this makes the film uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the roughness of the film is much too high, giving it a very matt appearance (very low gloss), undesirable for many applications.
EP-A 0 360 201 describes a polyester film having at least two layers and comprising a base layer with fine vacuoles, with a density of from 0.4 to 1.3 kg/dm3, and having at least one outer layer whose density is above 1.3 kg/dm3. The vacuoles are achieved by adding from 4 to 30% by weight of a crystalline propylene polymer, followed by biaxial stretching of the film. The additional outer layer improves the ease of production of the film (no streaking on the film surface), and the surface tension is increased and the roughness of the laminated surface can be reduced. A residual disadvantage is that regrind arising during production of the film (essentially a mixture of polyester and propylene homopolymer) cannot be reused without yellowing the film. However, this makes the process uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the roughnesses of the films listed in the examples are still too high, giving the films a matt appearance (low gloss), undesirable for many applications.
EP-A 0 795 399 describes a polyester film having at least two layers and comprising a base layer with fine vacuoles, the density of which is from 0.4 to 1.3 kg/dm3, and having at least one outer layer, the density of which is greater than 1.3 kg/dm3. The vacuoles are achieved by adding from 5 to 45% by weight of a thermoplastic polymer to the polyester in the base, followed by biaxial stretching of the film. The thermoplastic polymers used are, inter alia, polypropylene, polyethylene, polymethylpentene, polystyrene or polycarbonate, and the preferred thermoplastic polymer is polypropylene. As a result of adding the outer layer, ease of production of the film is improved (no streaking on the film surface), the surface tension is increased and the roughness of the laminated surface can be matched to prevailing requirements. Further modification of the film in the base layer and/or in the outer layers, using white pigments (generally TiO2) and/or using optical brighteners permits the properties of the film to be matched to the prevailing requirements of the application. A residual disadvantage is that regrind Which arises during production of the film (essentially a mixture of polyester and the added polymer) cannot be reused without undefined and highly undesirable changes in the color of the film. This makes the process uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the films listed in the examples continue to have excessive roughness values, giving them a matt appearance (low gloss), undesirable for many applications.
DE-A 195 40 277 describes a polyester film having one or more layers and comprising a base layer with fine vacuoles, with a density of from 0.6 to 1.3 kg/dm3, and having planar birefringence of from xe2x88x920.02 to 0.04. The vacuoles are achieved by adding from 3 to 40% by weight of a thermoplastic resin to the polyester in the base, followed by biaxial stretching of the film. The thermoplastic resins used are, inter alia, polypropylene, polyethylene, polymethylpentene, cyclic olefin polymers, polyacrylic resins, polystyrene or polycarbonate, preferred polymers being polypropylene and polystyrene. By maintaining the stated limits for the birefringence of the film, the film claimed has in particular superior tear strength and superior isotropy properties. However, a residual disadvantage is that regrind arising during production of the film cannot be reused without undefined discoloration of the film arising, and this in turn is highly undesirable. This makes the process uneconomic, but the film produced with regrind would not gain acceptance in the market. In addition, the roughnesses of the films listed in the examples are still too high, giving them a matt appearance (low gloss), undesirable for many applications.
DE-A 23 46 787 describes a flame-retardant polymer. Besides the polymer itself, its use for producing films and fibers is also described. However, the following shortcomings were apparent during production of films with this phospholane-modified polymer claimed in the DE-A:
The polymer is very sensitive to hydrolysis and has to be very thoroughly predried.
On drying with dryers of the prior art, the polymer coagulates, making it extremely difficult, or even impossible, to produce a film.
The films produced, under conditions which are extreme and not cost-effective, embrittle when exposed to heat, i.e. their mechanical properties deteriorate sharply due to the rapid onset of embrittlement, making the film industrially unusable. This embrittlement arises after as little as 48 hours of exposure to heat.
The object of the present invention was to provide a white, biaxially oriented polyester film which has high gloss and improved ease of production, i.e. low production costs, and which moreover has good heat resistance and flame retardancy. In particular, it should be possible for cut material (regrind) directly associated with the film production process to be reused in the production process at a concentration of from 10 to 70% by weight, based on the total weight of the film, without any resultant adverse effect on the physical or optical properties of the film produced with regrind. In particular, addition of regrind should not cause any significant yellowing of the film.
High flame retardancy means that in what is known as a fire protection test the white film meets the requirements of DIN 4102, Part 2 and in particular the requirements of DIN 4102, Part 1 and can be classified in building materials class B2, in particular B1, for low-flammability materials.
The film should moreover pass the UL 94 test known as the xe2x80x9cVertical burning test for flammability of plastic materialsxe2x80x9d, therefore qualifying for classification 94 VTM-0. This means that the film ceases to burn within 10 seconds after removal of the bunsen burner, that no further smoldering is observed after 30 seconds, and that no flaming drops are observed over the entire period.
Cost-effective production includes the capability of the polymers or polymer components needed for producing the flame-retardant film to be dried using industrial dryers of the prior art. It is important that the polymers do not cake and do not undergo thermal degradation. These industrial dryers of the prior art include vacuum dryers, fluidized-bed dryers, moving-bed dryers and fixed-bed dryers (power dryers). The dryers mentioned operate at temperatures of from 100 to 170xc2x0 C., at which flame-retardant polymers usually cake and have to be dug out, making film production impossible.
In vacuum dryers, which have the gentlest drying action, the polymer passes through a range of temperature of from about 30 to 130xc2x0 C. at a pressure of 50 mbar. A process known as postdrying is then required, in a hopper at temperatures of from 100 to 130xc2x0 C. and with a residence time of from 3 to 6 hours. Even here, the known polymer cakes to an extreme extent.
Good heat resistance means that the film and its mechanical properties do not deteriorate after 100 hours of annealing at 100xc2x0 C. in a circulating-air heating cabinet.
According to the invention, the object is achieved by means of a white, biaxially oriented, flame-retardant polyester film with at least one base layer made from polyester, the characterizing features of which are that at least the base layer also comprises, based on the weight of the base layer, from 2 to 60% by weight of a cycloolefin copolymer (COC), where the glass transition temperature of the cycloolefin copolymer (COC) is within the range from 70 to 270xc2x0 C., and that the film comprises at least one flame retardant which is preferably fed directly as a masterbatch to the polyester during film production.
The white, biaxially oriented polyester film as defined in the present invention is a film of this type whose whiteness is above 70%, preferably above 75% and particularly preferably above 80%. In addition, the opacity of the film of the invention is above 55%, preferably above 60% and particularly preferably above 65%.
To achieve the desired whiteness of the film of the invention, the amount of COC in the base layer should be above 2% by weight, otherwise the whiteness is below 70%. On the other hand, if the amount of COC is above 60% by weight, the film is no longer cost-effective to produce, since the process of orienting the film becomes unreliable.
It is also necessary for the glass transition temperature of the COC used to be above 70xc2x0 C. Otherwise, if the glass transition temperature of the COC used is below 70xc2x0 C., the polymer mixture is difficult to process, since it becomes difficult to extrude. The desired whiteness is lost and use of regrind gives a film with a tendency toward increased yellowness. On the other hand, if the glass transition temperature of the COC selected is above 270xc2x0 C. the homogenization of the polymer mixture in the extruder will no longer be sufficient. This then gives a film with undesirably inhomogeneous properties.
In the preferred embodiment of the film of the invention, the glass transition temperature of the COCs used is within the range from 90 to 250xc2x0 C., and in the particularly preferred embodiment it is within the range from 110 to 220 C.
Surprisingly, it has been found that a white, opaque, glossy film can be produced by adding a COC in the manner described above.
The whiteness and the opacity of the film can be adjusted with precision and adapted to particular requirements by varying the amount and nature of the COC added. This means that the use of other commonly used whitening or opacifying additives can substantially be dispensed with. It was also highly surprising that the surface roughness of the film is substantially lower, and therefore the gloss of the film substantially higher, than for comparable films of the prior art. A quite sensational discovery was the additional effect that, despite the presence of flame retardant, regrind exhibits no tendency toward yellowing, as is observed when using polymeric additives and conventional flame retardants of the prior art.
None of the features described was foreseeable. This was particularly the case since COC is evidently substantially incompatible with polyethylene terephthalate and is known to require stretching ratios and stretching temperatures similar to those for polyethylene terephthalate. Under these circumstances the skilled worker would not have expected that a white, opaque film with high gloss could be produced under these production conditions.

In the preferred and particularly preferred embodiments, the film of the invention has high/particularly high whiteness and high/particularly high opacity, while addition of regrind causes extremely little change in the color of the film.
The film of the invention comprises at least one flame retardant, which is fed by way of what is known as masterbatch technology directly during film production, the concentration of the flame retardant being within the range from 0.5 to 30.0% by weight, preferably from 1.0 to 20.0% by weight, based on the weight of the layer which comprises the flame retardant. During production of the masterbatch, the relationship between flame retardant and thermoplastic is generally within the range from 60% by weight:40% by weight to 10% by weight:90% by weight.
Typical flame retardants include bromine compounds, chloroparaffins and other chlorine compounds, antimony trioxide, and alumina trihydrates, the halogen compounds being disadvantageous since they produce halogen-containing byproducts. Other serious disadvantages are the low lightfastness of films in which these compounds are present, and the evolution of hydrogen halides in the event of a fire.
Examples of suitable flame retardants used according to the invention are organic phosphorus compounds, such as carboxyphosphinic acids, anhydrides thereof and dimethyl methylphosphonate.
Since the flame retardants generally have some degree of susceptibility to hydrolysis, concomitant use of a hydrolysis stabilizer may be advisable.
The hydrolysis stabilizers used are generally amounts of from 0.01 to 1.0% by weight of phenolic stabilizers, of alkali metal/alkaline earth metal stearates and/or of alkali metal/alkaline earth metal carbonates. The amounts of phenolic stabilizers used are preferably from 0.05 to 0.6% by weight, in particular from 0.15 to 0.3% by weight, and their molar mass is preferably above 500 g/mol. Particularly advantageous compounds are pentaerythrityl tetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.
It was therefore more than surprising that by using masterbatch technology and a suitable predrying and/or precrystallization procedure and, if desired, using small amounts of a hydrolysis stabilizer, it is possible to produce a flame-retardant, thermoformable film with the required property profile cost-effectively and especially without any caking in the dryer, and that on exposure to high temperature the film does not embrittle, and does not break when folded.
It was very surprising that, together with this excellent result and with the flame retardancy required:
the Yellowness Index of the film undergoes no adverse change when compared with that of an unstabilized film, within the bounds of accuracy of measurement;
there are no releases of gases, no die deposits and no frame condensation, and the film therefore has excellent optical properties and excellent profile and layflat;
the flame retardant, UV-resistant film has excellent stretchability, and therefore can be produced in a reliable and stable manner on high-speed film lines at speeds of 420 m/min.
The film is therefore also cost-effective.
The film of the invention has one or more layers. Single-layer embodiments have a structure like that of the COC-containing layer described below. Embodiments having more than one layer have at least two layers and always comprise the COC-containing layer and at least one other layer, where the COC-containing layer is the base layer but may also form the intermediate layer or the outer layer of the film having two or more layers. In one preferred embodiment, the COC-containing layer forms the base layer of the film with at least one outer layer and preferably outer layers on both sides, and an intermediate layer or intermediate layers may be present, if desired, on one or both sides. In another preferred embodiment, the COC-containing layer also forms an intermediate layer of the multilayer film. Other embodiments with COC-containing intermediate layers have a five-layer structure with COC-containing intermediate layers on both sides of the COC-containing base layer. In another embodiment, the COC-containing layer may form, as well as the base layer, an outer layer or outer layers on the base layer or intermediate layer, on one or both sides. For the purposes of the present invention, the base layer is that layer which makes up more than from 50 to 100%, preferably from 70 to 90%, of the total film thickness. The outer layer is always the layer which forms the outer layer of the film, and it is preferable for the invention if one or two outer layers have been arranged on the COC-containing base layer, and if the flame retardant is present in the outer layer(s).
Each embodiment of the invention is a non-transparent, white film. For the purposes of the present invention, non-transparent films are those films whose light transmittance to ASTM D1003-77 is below 95%, preferably below 75%.
The COC-containing layer (the base layer) of the film of the invention comprises a polyester, preferably a polyester homopolymer, a COC, the flame retardant, and also, if desired, other additives, in each case in effective amounts. This layer generally comprises at least 20% by weight, preferably from 40 to 98% by weight, in particular from 70 to 96% by weight, of polyester, based on the weight of the layer.
The base layer of the film comprises, as main constituent, a thermoplastic polyester. Polyesters suitable here are those made from ethylene glycol and terephthalic acid (=polyethylene terephthalate, PET), from ethylene glycol and naphthalene-2,6-dicarboxylic acid (=polyethylene 2,6-naphthalate, PEN), from 1,4-bishydroxymethylcyclohexane and terephthalic acid (=poly-1,4-cyclohexanedimethylene terephthalate, PCDT) or else from ethylene glycol, naphthalene-2,6-dicarboxylic acid and biphenyl4,4xe2x80x2-dicarboxylic acid (=polyethylene 2,6-naphthalate bibenzoate, PENBB). Particular preference is given to polyesters which are composed of at least 90 mol %, preferably at least 95 mol %, of ethylene glycol units and terephthalic acid units or ethylene glycol units and naphthalene-2,6-dicarboxylic acid units. The remaining monomer units are derived from other aliphatic, cycloaliphatic or aromatic diols and, respectively, dicarboxylic acids, as may also be present in layer A (A=outer layer 1) or in layer C (C=outer layer 2) of a multilayered ABC (B=base layer) film.
Examples of other suitable aliphatic diols are diethylene glycol, triethylene glycol, aliphatic glycols of the formula HOxe2x80x94(CH2)nxe2x80x94OH, where n is an integer from 3 to 6 (in particular 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol) or branched aliphatic glycols having up to 6 carbon atoms. Among the cycloaliphatic diols, mention should be made of cyclohexanediols (in particular 1,4-cyclohexanediol). Other suitable aromatic diols are those, for example, of the formula HOxe2x80x94C6H4xe2x80x94Xxe2x80x94C6H4xe2x80x94OH where X is xe2x80x94CH2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94S02xe2x80x94. Bisphenols of the formula HOxe2x80x94C6H4xe2x80x94C6H4xe2x80x94OH are also highly suitable.
Other preferred aromatic dicarboxylic acids are benzenedicarboxylic acids, naphthalenedicarboxylic acids (such as naphthalene-1,4- or -1,6-dicarboxylic acid), biphenyl-x,xxe2x80x2-dicarboxylic acids (in particular biphenyl-4,4xe2x80x2-dicarboxylic acid), diphenylacetylene-x,xxe2x80x2-dicarboxylic acids (in particular diphenylacetylene4,4xe2x80x2-dicarboxylic acid) and stilbene-x,xxe2x80x2-dicarboxylic acids. Among the cycloaliphatic dicarboxylic acids mention should be made of cyclohexanedicarboxylic acids (in particular cyclohexane-1,4-dicarboxylic acid). Among the aliphatic dicarboxylic acids, the (C3-C19)-alkanedioic acids are particularly suitable, where the alkane moiety may be straight-chain or branched.
The polyesters may, for example, be prepared by the transesterification process. The starting materials here are dicarboxylic esters and diols, and these are reacted using the usual transesterification catalysts, such as salts of zinc, of calcium, of lithium, of magnesium or of manganese. The intermediates are then polycondensed in the presence of typical polycondensation catalysts, such as antimony trioxide or titanium salts. They may equally well be prepared by the direct esterification process in the presence of polycondensation catalysts, starting directly from the dicarboxylic acids and the diols.
According to the invention, the COC-containing layer (base layer) or, in the case of single-layer embodiments, the film, comprises an amount of not less than 4.0% by weight, preferably from 5 to 50% by weight and particularly preferably from 6 to 40% by weight, of a cycloolefin copolymer (COC), based on the weight of the base layer or, in the case of single-layer embodiments, based on the weight of the film. It is significant for the present invention that the COC is not compatible with the polyethylene terephthalate and does not form a homogeneous mixture with the same.
Cycloolefin polymers are homopolymers or copolymers which contain polymerized cycloolefin units and, if desired, acyclic olefins as comonomer. Cycloolefin polymers suitable for the present invention contain from 0.1 to 100% by weight, preferably from 10 to 99% by weight, particularly preferably from 50 to 95% by weight, of polymerized cycloolefin units, in each case based on the total weight of the cycloolefin polymer. Particular preference is given to polymers which have been built up using the monomers comprising the cyclic olefins of the formulae I, II, III, IV, V or VI:
R1, R2, R3, R4, R5, R6, R7 and R8 in these formulae are identical or different and are a hydrogen atom or a C1-C30-hydrocarbon radical, or two or more of the radicals R1 to R8 have been bonded cyclically, and the same radicals in the different formulae may have the same or a different meaning. Examples of C1-C30-hydrocarbon radicals are linear or branched C1-C8-alkyl radicals, C6-C18-aryl radicals, C7-C20-alkylenearyl radicals and cyclic C3-C20-alkyl radicals and acyclic C2-C20-alkenyl radicals.
If desired, the COCs may contain from 0 to 45% by weight, based on the total weight of the cycloolefin polymer, of polymerized units of at least one monocyclic olefin of the formula VIl: 
n here is a number from 2 to 10.
If desired, the COCs may contain from 0 to 99% by weight, based on the total weight of the COC, of polymerized units of an acyclic olefin of the formula VIII: 
R9, R10, R11 and R12 here are identical or different and are a hydrogen atom or a C1-C10-hydrocarbon radical, e.g. a C1-C8-alkyl radical or a C6-C14-aryl radical.
Other polymers suitable in principle are cycloolefin polymers which are obtained by ring-opening polymerization of at least one of the monomers of the formulae I to VI, followed by hydrogenation.
Cycloolefin homopolymers have a structure composed of one monomer of the formulae I to VI. These cycloolefin polymers are less suitable for the purposes of the present invention. Polymers suitable for the purposes of the present invention are cycloolefin copolymers (COC) which comprise at least one cycloolefin of the formulae I to VI and acyclic olefins of the formula VIII as comonomer. Acyclic olefins preferred here are those which have from 2 to 20 carbon atoms, in particular unbranched acyclic olefins having from 2 to 10 carbon atoms, for example ethylene, propylene and/or butylene. The proportion of polymerized units of acyclic olefins of the formula VIII is up to 99% by weight, preferably from 5 to 80% by weight, particularly preferably from 10 to 60% by weight, based on the total weight of the respective COC.
Among the COCs described above, those which are particularly preferred contain polymerized units of polycyclic olefins having a fundamental nor-bornene structure, particularly preferably norbornene or tetracyclododecene. Particular preference is also given to COCs which contain polymerized units of acyclic olefins, in particular ethylene. Particular preference is in turn given to norbornene-ethylene copolymers and tetracyclododecene-ethylene copolymers which in each case contain from 5 to 80% by weight, preferably from 10 to 60% by weight, of ethylene (based on the weight of the copolymer).
The cycloolefin polymers generically described above generally have glass transition temperatures Tg in the range from xe2x88x9220 to 400xc2x0 C. However, COCs which can be used for the invention have a glass transition temperature Tg above 70xc2x0 C., preferably above 90xc2x0 C. and in particular above 110xc2x0 C. The viscosity number (decalin, 135xc2x0 C., DIN 53 728) is advantageously from 0.1 to 200 ml/g, preferably from 50 to 150 ml/g.
The COCs are prepared by heterogeneous or homogeneous catalysis with organometallic compounds, as described in a wide variety of documents. Suitable catalyst systems based on mixed catalysts made from titanium compounds and, respectively, vanadium compounds in conjunction with aluminum organyl compounds are described in DD 109 224, DD 237 070 and EP-A-0 156 464. EP-A-0 283 164, EP-A-0 407 870, EP-A-0 485 893 and EP-A-0 503 422 describe the preparation of COCs with catalysts based on soluble metallocene complexes. The preparation processes for COCs described in the abovementioned specifications are expressly incorporated herein by way of reference.
The COCs are incorporated into the film either in the form of pure granules or in the form of granulated concentrate (masterbatch), by premixing the polyester granules or polyester powder with the COC or, respectively, with the COC masterbatch, followed by feeding to an extruder. In the extruder, the mixing of the components continues and they are heated to the processing temperature. It is advantageous here for the novel process if the extrusion temperature is above the glass transition temperature Tg of the COC, generally above the glass transition temperature of the COC by at least 5 K, preferably by from 10 to 180 K, in particular by from 15 to 150 K.
For the intermediate layers and for the outer layers, it is possible in principle to use the polymers used for the base layer. Besides these, other materials may also be present in the outer layers, and the outer layers are then preferably composed of a mixture of polymers or of a copolymer or of a homopolymer which comprise ethylene 2,6-naphthalate units and ethylene terephthalate units. Up to 30 mol % of the polymers may be composed of other comonomers (e.g. ethylene isophthalate units).
The base layer and the other layers may additionally comprise conventional additives, such as stabilizers, antiblocking agents and other fillers. They are advantageously added to the polymer or, respectively, to the polymer mixture prior to melting. Examples of stabilizers used are phosphorus compounds, such as phosphoric acid or phosphoric esters.
Typical antiblocking agents (in this context also termed pigments) are inorganic and/or organic particles, such as calcium carbonate, amorphous silica, talc, magnesium carbonate, barium carbonate, calcium sulfate, barium sulfate, lithium phosphate, calcium phosphate, magnesium phosphate, aluminum oxide, lithium fluoride, the calcium, barium, zinc or manganese salts of the dicarboxylic acids used, carbon black, titanium dioxide, kaolin or crosslinked polymer particles, e.g. polystyrene particles or acrylate particles.
The additives selected may also comprise mixtures of two or more different antiblocking agents or mixtures of antiblocking agents of the same composition but different particle sizes. The particles may be added to the polymers of the individual layers of the film in the respective advantageous amounts, e.g. as a glycolic dispersion during the polycondensation or via masterbatches during extrusion. Pigment concentrations which have proven particularly suitable are from 0 to 25% by weight (based on the weight of the respective layer). EP-A-0 602 964, for example, describes the antiblocking agents in detail.
To improve the whiteness of the film, the base layer or the other additional layers may comprise further pigmentation. It has proven particularly advantageous here for the additional materials added to be barium sulfate with a particle size of from 0.3 to 0.8 xcexcm, preferably from 0.4 to 0.7 xcexcm, or titanium dioxide with a particle size of from 0.05 to 0.3 xcexcm. This gives the film a brilliant white appearance. The concentration of barium sulfate or titanium dioxide is within the range from 1 to 25% by weight, preferably from 1 to 20% by weight, and very preferably from 1 to 15% by weight.
The total thickness of the film may vary within wide limits and depends on the application envisaged. The preferred embodiments of the novel film have total thicknesses of from 4 to 400 xcexcm, preferably from 8 to 300 xcexcm, particularly preferably from 10 to 300 xcexcm. The thickness of any intermediate layer(s) present is/are, in each case independently of one another, from 0.5 to 15 xcexcm, preferably from 1 to 10 xcexcm, in particular from 1 to 8 xcexcm. All the values given are based on one intermediate layer. The thickness of the outer layer(s) is selected independently of the other layers and is preferably within the range from 0.1 to 10 xcexcm, in particular from 0.2 to 5 xcexcm, preferably from 0.3 to 2 xcexcm, and outer layers applied on both sides may be identical or different in terms of their thickness and composition. The thickness of the base layer is therefore given by the difference between the total thickness of the film and the thickness of the outer and intermediate layer(s) applied, and, similarly to the total thickness, may therefore vary within wide limits.
In one particular embodiment, the outer layers may also be composed of a polyethylene naphthalate homopolymer, or of an ethylene terephthalate-ethylene naphthalate copolymer, or of a compound.
In this embodiment, the standard viscosity of the thermoplastics of the outer layers is similar to that of the polyethylene terephthalate of the base layer.
In the embodiments having two or more layers, the flame retardant is preferably present in the base layer. However, the outer layers may, if required, also have flame retardant.
In another embodiment, flame retardant may be present in the outer layers. If required, or if fire-protection requirements are stringent, the base layer may additionally comprise what is known as a base level of flame retardant.
Unlike in the single-layer embodiment, the amount of flame retardant here in percent by weight is based on the weight of the respective layer having the agents.
It is also surprising that fire tests to DIN 4102 Part 1 and Part 2, and also the UL 94 test, have shown that films of the invention comply with the requirements.
The flame-retardant films having two or more layers and produced by known coextrusion technology are therefore of great interest in economic terms when compared with monofilms provided with full flame retardancy, since markedly less additives are needed to achieve comparable flame retardancy.
During production of the film it was found that the flame-retardant film gives excellent longitudinal and transverse orientation without break-offs. In addition, no gas releases of any type attributable to the presence of flame retardant were found, and this is important for the invention, since most conventional flame retardants evolve very undesirable and unpleasant gases, attributable to the decomposition of these compounds under the conditions of processing, at extrusion temperatures above 260xc2x0 C., and are therefore of no use.
Surprisingly, even films of the invention in the range of thickness from 5 to 300 xcexcm comply with requirements for the construction materials class B1 to DIN 4102 Part 1 and with those for the UL 94 test.
During production of the white, flame-retardant film it was also found that the flame retardant can be incorporated using masterbatch technology and suitable predrying and/or precrystallization of the flame retardant masterbatch without the occurrence of caking in the dryer, and therefore the film can be produced cost-effectively.
It was more than surprising that incorporation is made even easier by small additions of a hydrolysis stabilizer within the flame retardant masterbatch. Throughputs, and therefore production rates, could readily be increased in this way. In one very specific embodiment, the film also comprises small amounts of a hydrolysis stabilizer in the layers having flame retardant.
Measurements showed that the film of the invention does not embrittle over long periods at high temperatures of 100xc2x0 C., a fact which is more than surprising. This result is attributable to the synergistic action of suitable precrystallization, predrying, masterbatch technology and provision of flame retardant.
The film of the invention can moreover readily be recycled without pollution of the environment and without loss of mechanical properties, and examples of uses for which it is suitable are therefore short-lived promotional placards for constructing exhibition stands and other promotional requisites where fire protection is desirable.
The invention further provides a process for producing the polyester film of the invention by the extrusion or coextrusion process known per se.
According to the invention, the flame retardant, with or without the hydrolysis stabilizer, is fed by way of masterbatch technology. The flame retardant is fully dispersed in a carrier material. Carrier materials which may be used are the polyester itself, e.g. the polyethylene terephthalate, or else other polymers compatible with the polyester.
It is important in masterbatch technology that the particle size and the bulk density of the masterbatch are similar to the particle size and the bulk density of the polyester, so that homogeneous distribution is achieved, giving uniform stabilization.
The polyester films may be produced by known processes from polyester with, if desired, other polymers, with the flame retardant, with or without the hydrolysis stabilizer and/or with other customary additives in customary amounts of from 1.0 to a maximum of 30% by weight, either in the form of a monofilm or else in the form of, if desired coextruded, films having two or more layers and with identically or differently constructed surfaces, where one surface may have been pigmented, for example, but no pigment is present at the other surface. Known processes may also have been used to provide one or both surfaces of the film with a conventional functional coating.
It is important for the invention that the masterbatch which comprises the flame retardant and, if used, the hydrolysis stabilizer, is precrystallized or predried. This predrying includes gradual heating of the masterbatch at reduced pressure (from 20 to 80 mbar, preferably from 30 to 60 mbar, in particular from 40 to 50 mbar) with agitation, and, if desired, post-drying at a constant, elevated temperature, again at reduced pressure. It is preferable for the masterbatch to be charged at room temperature from a metering vessel in the desired blend together with the polymer of the base and/or outer layers and, if desired, with other raw material components batchwise into a vacuum dryer in which the temperature profile moves from 10 to 160xc2x0 C., preferably from 20 to 150xc2x0 C., in particular from 30 to 130xc2x0 C., during the course of the drying time or residence time. During the residence time of about 6 hours, preferably 5 hours, in particular 4 hours, the raw material mixture is stirred at from 10 to 70 rpm, preferably from 15 to 65 rpm, in particular from 20 to 60 rpm. The resultant precrystallized or predried raw material mixture is post-dried in a downstream vessel, likewise evacuated, at temperatures of from 90 to 180xc2x0 C., preferably from 100 to 170xc2x0 C., in particular from 110 to 160xc2x0 C., for from 2 to 8 hours, preferably from 3 to 7 hours, in particular from 4 to 6 hours.
For the coextrusion process, the procedure is that the melt(s) corresponding to the single-layer film or to the individual layers of the film is/are extruded/coextruded through a flat-film die, the resultant film is drawn off for solidification on one or more rolls, the film is then biaxially stretched (oriented), and the biaxially stretched film is then heat-set and, if desired, corona- or flame-treated on the surface layer intended for further treatment.
The biaxial orientation is generally carried out in succession. For this, it is preferable to orient first longitudinally (i.e. in MD, the machine direction) and then transversely (i.e. in TD, perpendicularly to the machine direction). This orientates the molecular chains. The longitudinal orientation preferably takes place with the aid of two rolls rotating at different rates corresponding to the desired stretching ratio. For the transverse stretching, an appropriate suitable tenter frame is generally used.
Simultaneous orientation of the film of the invention in the two directions (MD and TD) with the aid of a tenter frame suitable for this purpose has proven not to be appropriate, since this stretching method gives a film which has insufficient whiteness and insufficient opacity.
The temperature at which the orientation is carried out may be varied over a relatively wide range and depends on the properties desired in the film. In general, the longitudinal stretching is carried out at from 80 to 130xc2x0 C. and the transverse stretching at from 90 to 150xc2x0 C. The longitudinal stretching ratio is generally within the range from 2.5:1 to 6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio is generally within the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to 4.5:1.
In the heat-setting which follows, the film is held at a temperature of from 150 to 250xc2x0 C. for from about 0.1 to 10 s. The film is then cooled and then wound up in the usual manner.
To establish other desired properties, the film may be chemically treated or else corona- or, respectively, flame-treated. The intensity of treatment is selected such that the surface tension of the film is generally above 45 mN/m.
To establish other properties, the film may also be coated. Typical coatings have adhesion-promoting, antistatic, slip-improving or release action. It is clear that these additional coatings may be applied to the film by in-line coating using aqueous dispersions, prior to the transverse stretching procedure.
The particular advantage of the novel film is its high whiteness and high opacity, together with good flame retardancy. Surprisingly, the gloss of the film was also very high. The whiteness of the film is above 70%, preferably above 75% and particularly preferably above 80%. The opacity of the novel film is above 55%, preferably above 60% and particularly preferably above 65%. The gloss of the novel film is above 80, preferably above 90 and particularly preferably above 100.
Another particular advantage of the invention is that regrind material produced directly during the production process can be reused at a concentration of from 10 to 70% by weight, based on the total weight of the film, without any significant negative effect on the physical properties of the film. In particular, the regrinded material (composed essentially of polyester and COC) does not give undefined changes in the color of the film, as is the case in the films of the prior art.
A further advantage of the invention is that the production costs of the novel film are comparable with those of conventional opaque films of the prior art. The other properties of the novel film relevant to its processing and use remain essentially unchanged or are even improved.
The film has excellent suitability for packing foods or other consumable items which are sensitive to light and/or to air. It is also highly suitable for use in the industrial sector, e.g. for producing hot-stamping foils or as a label film. Besides this, the film is, of course, particularly suitable for image-recording papers, printed sheets, magnetic recording cards, to name just a few possible applications.
The processing performance and winding performance of the film, in particular on high-speed machines (winders, metallizers, printing machines and laminating machines) is exceptionally good. A measure of processing performance is the coefficient of friction of the film, which is below 0.6. A decisive factor affecting winding performance, besides a good thickness profile, excellent layflat and a low coefficient of friction, is the roughness of the film. It has become apparent that the winding of the novel film is particularly good if the average roughness is within the range from 50 to 250 nm while the other properties are complied with. The roughness may be varied within the stated range by, inter alia, varying the COC concentration and the process parameters in the production process.
The most important film properties according to the invention are again summarized at a glance in the table below (Table 1), thus providing a particularly clear picture.
The following values were measured to characterize the polymers and the films:
SV (DCA), IV (DCA)
The standard viscosity SV (DCA) is measured in dichloroacetic acid by analogy with DIN 53726.
The intrinsic viscosity (IV) is calculated as follows from the standard viscosity (SV)
IV(DCA)=6.67xc2x710-4 SV(DCA)+0.118
Yellowness Index
The Yellowness Index YID is the deviation from the colorless state in the xe2x80x9cyellowxe2x80x9d direction and is measured to DIN 6167.
Fire Performance
Fire performance is determined to DIN 4102, Part 2, construction materials class B2, and to DIN 4102, Part 1, construction materials class B1, and also by the UL 94 test.
Coefficient of Friction
Coefficient of friction is determined to DIN 53 375. The coefficient of sliding friction was determined 14 days after production.
Surface Tension
Surface tension was determined by what is known as the ink method (DIN 53 364).
Roughness
Roughness Ra of the film was determined to DIN 4768 with a cut-off of 0.25 mm.
Whiteness and Opacity
Whiteness and opacity were determined with the aid of a Zeiss, Oberkochem (DE) xe2x80x9cELREPHOxe2x80x9d electric reflectance photometer, standard illuminant C, 2xc2x0 normal observer. Opacity is determined to DIN 53 146. Whiteness is defined as W=RY+3RZxe2x88x923RX. W=whiteness, RY, RZ and RX=relevant reflection factors when the Y, Z and X color-measurement filter is used. The white standard used was a barium sulfate pressing (DIN 5033, Part 9). A detailed description is given, for example, in Hansl Loos xe2x80x9cFarbmessungxe2x80x9d [Color measurement], Verlag Beruf und Schule, Itzehoe (1989).
Light Transmittance
Light transmittance is measured using a method based on ASTM D1033-77.
Gloss
Gloss was determined to DIN 67 530. The reflectance was measured as an optical value characteristic of a film surface. Based on the standards ASTM-D 523-78 and ISO 2813, the angle of incidence was set at 60xc2x0. A beam of light hits the flat test surface at the set angle of incidence and is reflected and/or scattered by this surface. A proportional electrical variable is displayed representing light beams hitting the photo-electronic detector. The value measured is dimensionless and must be stated together with the angle of incidence.
Glass Transition Temperature
The glass transition temperature Tg was determined using film specimens with the aid of DSC (differential scanning calorimetry) (DIN 73 765). A DuPont DSC 1090 was used. The heating rate was 20 K/min and the specimen weight was about 12 mg. The glass transition Tg was determined in the first heating procedure. Many of the specimens showed an enthalpy relaxation (a peak) at the beginning of the step-like glass transition. The temperature taken as Tg was that at which the step-like change in heat capacityxe2x80x94without reference to the peak-shaped enthalpy relaxationxe2x80x94achieved half of its height in the first heating procedure. In all cases, there was only a single glass transition observed in the thermogram in the first heating procedure.